Halogen compounds hypervalent

Halogen cations, 319 hexafluoro, 316 tetrafluoro, 315 Halogen compounds hypervalent, 316... 

An extensive review55 of the chemistry of halogen complexes containing carbon ligands, the hypervalent halogen compounds , has been published recently. This is written from an organic chemical viewpoint and includes discussion of the organic chemistry involved in reactions of these compounds. Only aspects directly related to coordination chemistry will be mentioned here. 

Koser GF (1983) Hypervalent Halogen Compounds. In Patai S, Rappoport Z (eds) The Chemistry of Functional Groups, Supplement D. Wiley, New York, p 721, chap 18... 

G.F. Koser, Hypervalent halogen compounds, in The Chemistry of Functional Groups, Suppl. D, eds S. Patai and Z. Rappoport, Chapter 18. Wiley-Interscience, Chichester, 1983. 

The intent of this review is not to cover all aspects of the use of halides in synthesis but rather to cover recent developments that are hopefully of interest to the synthetic community at large. Other chapters in the present volume cover some of the reactions of organic halides that are mediated by transition metal compounds (by J. Green) and the use of hypervalent halogen compounds (by G. Koser). The reader is referred to these chapters for further detail. 

Only four chapters which were planned and promised for the present volume did not materialize. These were the following Acidity, basicity, H-bonding and complex formation Hypervalent halogen compounds Toxicology and pharmacology and Perfluorocarbons . 

Halogenated aromatic compounds may also be oxidized by CYP monooxygenases, yielding hypervalent halogenated compounds. 

Heteroatoms such as nitrogen or sulfur are oxidized on their free peripheric electrons (Fig. 32.12) as described for thiophene. Halogenated aromatic compounds, may also be oxidized by cytochrome P-450 monooxygenases, yielding hypervalent halogenated compounds. 

Synthesis, reactivity, and applications of sulphur- selenium-, and tellurium-halogen compounds electronic structure and bonding of hypervalent compounds of the heavy main group elements... 

Rather selective and mild oxidizing agents, namely hypervalent halogen compounds or their precursors (e.g., iodic acid, o-iodoxybenzoic acid (IBX), NaBr02, NaBr03, etc.), have been widely introduced into the field... 

Koser, G.F., Hypervalent halogen compounds, in The Chemistry ofPunaional Groups, Supplement D, Patai, S. and Rappoport, Z., Eds., John Wiley Sons, New York, 1983, part 1, chap. 18. 

When BrF3 is mixed with bromine, the active species is bromine monofluoride and this can be added to olefins (equation 118)213. Bromine trifluoride also used to prepare compounds with hypervalent heteroatoms, many of them, unattainable otherwise (equation 119)214. A key step in Lemal and coworkers unique synthesis of perfluorocyclopentadiene involved also an oxidative fluorination (equation 120)215. BrF3 can also substitute halogens and was used for the preparation of L-3-fluoroalanine 21 (equation 121)216. 

Chemical oxidation of silyl enolates has been performed with a variety of inorganic and organic oxidants such as ozone, copper(II) salts, Pb(OAc)4, Ag20, hypervalent iodine compounds such as iodosobenzene in methanol, (NH4)2[Ce(N03)6], xenon difluoride, tetranitromethane, halogens, nitronium-, diazonium- and triphenylmethyl salts, chloranil and ddq. 

The condensation of two molecules of primary selenoamides proceeds with bromine under extrusion of selenium to give selenadiazoles [93]. The reaction of selenocarbonates, selenothiocarbonates [94], and selenourea [95] with bromine and iodine has been widely tested. The products depend on the amount of halogen used. For example, in the reaction of N-methylthiazoline-2(3H)-selone, the use of one equivalent of bromine gives hypervalent lO-Se-3 complexes 44, whereas two equivalents of bromine cleave the carbon selenium double bond to give product 45. A similar hypervalent compound is formed from 4-imidazolin-2-selone,but the iodination of bis(imidazolin-2-selone)methane gives iodinated product 46. The availability of some of halogen adducts has been tested as a conducting material. 

The oxidation of thiols follows a completely different course as compared with the oxidation of alcohols, because the capacity of the sulfur atom to form hypervalent compounds allows it to become the site of oxidation. Thiols are readily oxidised to disulfides by mild oxidants such as atmospheric oxygen, halogens or iron(III) salts (Scheme 6). This type of reaction is unique to thiols and is not undergone by alcohols, it is a consequence of the lower bond strength of the S-H as compared with the O-H bond, so that thiols are oxidised at the weaker S-H bonds, whereas alcohols are preferentially oxidised at the weaker C-H bonds (Scheme 7). The mechanism of oxidation of thiols may be either radical or polar or both (Scheme 6). The polar mechanism probably involves transient sulfenic acid intermediates like (7) and (8). In contrast, thiols react with more powerful oxidants, like potassium permanganate, concentrated nitric acid or hydrogen peroxide, to yield the corresponding sulfonic acids (10). This oxidation probably proceeds via the relatively unstable sulfenic (7) and sulfinic acids (9), which are too susceptible to further oxidation to be isolated (Scheme 8). 

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